It is not a matter of how Roberts will vote, it is the nature of the case that will make it an emergent matter.
As far as the result, of course no one can predict with any certainty the vote of any Justice, but the fact is that the law passed on this subject is clear: the President has both the power to unilaterally declare an emergency related to foreign threats to our nation, and to take funds from the DoD to pay for construction deemed essential to help positively resolve the emergency.
Laugh all you want, it won’t change the law.
Courts will decide the degree of urgency. Not you or me or Trump.
PS he will be arguing emergency when he has done nothing for over 2 years.